Temperature compensated transistor amplifier circuit including fail-safe means



Dec. 14, 1965 3,223,934

P. H. L TEMPERATURE COMPENSATED TRANSISTOR AMPLIFIER CIRCUIT INCLUDING FAIL-SAFE MEANS Original Filed Deo. 5, 1958 2 Sheets-Sheet l Z500 (IPS. 125 0/ 10006253 [500 PS'. b 130 l5@ @Eid m2o I5 ESR W93 N ELCE'l/v fa L j 4// P12 'a' W2/Tok v L p a [Tazas/ 005 05,22 Cewen Dec. 14, 196s I UFT ,223,934

P. H. 3 TEMPERATURE COMPENSATED TRANSISTOR AMPLIFIER CIRCUIT INCLUDING FAIL-SAFE MEANS Original Filed Dec. 5. 1958 2 Sheets-Sheet 2 Zelay. lla/2 JNVENToR.

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United States Patent O 3,223,934 TEMPERATURE COMPENSATED TRANSS'IOR AMPLIFEER CIRCUIT INCLUDING FAIL-SAFE MEANS Philip H. Luft, Freeport, Ill., assigner to Westinghouse Air Brake Company, Wilmerding, lPa., a corporation of Pennsylvania Original application Dec. 3, 1958, Ser. No. "778,022, now Patent No. 3,035,167, dated May 1S, 1962. Divided and this application Nov. 8, 1961, Ser. No. 151,037

7 Claims. (Cl. S30- 15) My invention relates to railway track circuits, and more particularly to a railway track circuit including new and improved transmitter and receiver units which provide a means of positively defining the limits of said track circuit.

The present application is a division of my copending application for Letters Patent of the United States, Serial No. 778,022, led December 3, 1958, for Railway Track Circuit, now Patent No. 3,035,167 said copending application being a continuation-in-part of my copending application Serial No. 718,557 led on March 3, 1958, for Control Circuits now Patent No. 3,046,392 and assigned to the same assignee as this application.

Track circuits for use in conjunction with continuous track rails, that is, track rails having no insulating joints which track circuits have positively defined limits are in particular demand in the railway art for various applications such as railway crossing indicating systems and other superimposed track circuits for localized train detection. One type of superimposed or overlay track circuit with which I am familiar comprises a transmitter connected across the rails, and receiver leads also connected across the rails at a predetermined distance from the transmitter connections. It has been found that as the length of the above type track circuit increases, that is the distance between the transmitter and its associated receiver increases, its operating limits become less clearly defined. The operating limits of track circuits above 2500 feet have heretofore been almost impossible to detine. One reason for this lack of definition is the large variance in the impedance values of the track ballast due to climatic conditions. For example, the shunt detecting area for a track circuit operating at audio frequency and having the transmitter and receiver connections separated by 150 feet might vary only from 10 feet to 20 feet. However, the shunt detecting area of a similar track circuit having its transmitter and receiver connections separated by a distance of 4000 feet would vary from about 20 feet to a maximum of several hundred feet beyond the points where the receiver connects to the rails. Thus it can be seen that the greater the length of the track circuit, the greater the possible variance in shunt detecting area. Such a degree of variance undesirably affects a number of operations where the shunt or control area must be clearly dened, such as for example, highway crossing warning systems.

Heretofore adequate definition of the limits of the track circuits has been hampered by the characteristics of the transmitters and receivers used in the track circuits.

Accordingly, it is a principal object of my invention to provide a track circuit including a new and improved transmitter for providing constant and clearly defined track circuit limits.

It is another object of my invention to provide a track circuit including a new and improved receiver for provid-ing constant and clearly defined track circuit limits.

It is another object of my invention to provide an improved transmitter for compensation for variations in the load into which it feeds.

It is another object of this invention to provide a new and improved temperature compensated receiver.

3,223,934 Patented Dec. 14, 1965 ice In the attainment of the foregoing objects I provide a track circuit including a transmitter of signal energy operating at a given frequency and having a regulation curve which initially peaks, and then Ybecomes inversely proportional to the ballast or load impedance existing across the rails. The signal energy from the transmitter is coupled through the track rails to a sensitive receiver tuned to the frequency at which said transmitter is operating for controlling the operation of a track relay.

Other objects and advantages of my invention will become apparent from the following description and the accompanying drawings in which:

FIG. la is a view showing one embodiment of a track circuit to provide a bi-directional highway crossing control arrangement in accordance with my invention and showing one means of coupling signal energy from the rails to the receiver;

FIG. 1b is a view showing a second means of coupling signal energy from the rail to a receiver;

FIG. 2 is a schematic diagram of a transmitter employed in my track circuits;

FIG. 3 is a schematic diagram of a receiver employed in my track circuits;

FIGS. 4a, 4b and 4c are graphical illustrations useful in describing the operation of my track circuit.

I shall rst describe one embodiment of my track circuit including the new transmitter and receiver in accordance with my invention and shall then point out the novel features thereof in the appended claims.

It will be understood at the outset that a track circuit according to my invention may be employed in any application where it is desired to actuate a device when a train approaches or recedes a minimum number of feet from a designated point. However, my track circuit will be described only as applied to a highway crossing warning system since it is believed such a description is suicient for the understanding of the operation of my circuit.

Referring to FIG. 1a, a section of track rails 11a and 11b is shown as intersected by a highway crossing 14. In one practical embodiment, my track circuit includes a tirst transmitter 10 connected by leads 13a and 13b across rails 11a and 11b, respectively, at a desired point, say between 2500 and 4000 feet distance to the left, as oriented in the drawings, of the highway crossing. In one practical embodiment, transmitter 10 is tuned to operate at 1000 c.p.s.

A loop of wire 16 inductively coupled to rails 11a and 11b is placed between said rails on the other or right side of the highway crossing and is connected to the input circuit of receiver 20. Loop 16 is positioned such that a portion of the loop is adjacent a predetermined equal length of each of the rails 11a and 11b, and spaced approximately the same distance from each rail. As is obvious the sketch of loop 16 in the figures is not to scale. The left hand end of loop is placed at a designated distance, which may be a minimum distance of l5 feet from' the center of the highway to provide adequate clearance, that is to assure that a railway vehicle is completely oft" the highway before its rear wheels affect the operation of the loop as will hereinafter be described. Loop 16 eX- tends along rails 11a and 11b for a designated distance, in one embodiment about feet, to insure satisfactory signal coupling between the rails and the loop. For purposes of simplifying the following description, the eX- treme left and right hand ends of loop 16 are designated as leads 16a and 16b respectively. Receiver 20 is tuned to the operating frequency of transmitter 10 and operates in conjunction therewith.

A second transmitter 15, similar to transmitter 10, is connected by terminal leads 17a and 17b to rails 11a and 11b, respectively, at a desired point, say between 2500 `and 4000 feet distance to the right of highway crossing 14. In one practical embodiment, transmitter 15 is tuned to operate at 1500 c.p.s. Asecond loop 18 similar to loop 16 is positioned at a designated distance, a minimum of 15 feet on the left side of the highway crossing 14 and couples signal energy from ra-ils 11a and 11b to a receiver 25. As above, the extreme left and right h-and ends of loop 18 are designated as leads 1811 and 18a, respectively. Receiver 25 is tuned to the operating frequency of transmitter 15 and operates in conjunction therewith.

If the track circuits do not exceed 2500 feet in length, that is the distance between a transmitter and the associated receiver does not exceed 2500 feet, the receivers may be connected to the rails in the usual manner as shown in FIG. lb and the limits of the track circuit will be adequately defined. However, it has been found by tests that for track circuits of long lengths, say from 2500 feet to 4000 feet, it is necessary to inductively couple the signal energy to the receivers 20 and 25 through loops 16 and 18 respectively in order that the track circuit limits still be constant and clearly defined.

It has also been found that loop coupling eliminates interference to the receivers 20 and 25 in, for example, electric propulsion territory where the high harmonics of the propulsion current may interfere with the satisfactory operation of the control circuit of my invention. Since the propulsion current normally ows in the same direction along both rails, the voltage induced in loop 16 due to the propulsion current flowing in rail 11a will be in opposing or bucking relation to the voltage induced in loop 16 due to the propulsion current flowing in rail 11b. Thus, the voltages developed in loop 16 due to propulsion current cancel out and will not affect operation of my track circuit. However, each of transmitters and 15 provide current flow in rails 11a and 11b in opposite directions, hence the voltages induced in each portion of the loops will tend to aid or add and provide a high signal voltage to the associated receiver.

It will be understood that the distance referring to the points at which the electrical connections are made are given only as representative examples and are not intended as limiting in any way. The distance from one receiver, essentially located at the highway crossing, to the point at which the associated transmitter connects to the rails is limited only by the -available transmitting power while the length of the loop is determined by receiver sensitivity.

The means for coupling the signal energy of the transmitters to the rails will now be briefly described. Referring to FIG. 2, the output circuit for each transmitter includes the secondary winding 24 of output transformer 27 connected in series and tuned to filter 21 which is tuned to the operating frequency of the associated transmitter. One terminal of secondary winding 24 is connected to rail 11a by lead 13a and the other terminal thereof is connected to filter 21 which consists of the prim-ary winding 23 of a feedback transformer 28 in series with a capacitor 22. The other terminal of capacitor 22 is connected to rail 11b by lead 13b. A resistor or resistance 78 is connected across primary winding 23 for purposes herinafter described. The terms resistor and resistance are herein used interchangeably.

As noted above, transmitters 10 and 15, and receivers 20 and 25 -are similar. Transmitter 10 and associated receiver 20 are tuned to operate at 1000 c.p.s., while transmitter and associated receiver 25 are tuned to operate at 1500 c.p.s. Filter 21 and secondary winding 24 in one transmitter, although tuned to the operating frequency of their associated transmitter, are so designed that their impedance to the frequency at which the other transmitter is transmitting signal current energy is still of a low order. For example, even if the ballast impedance becomes infinite, filter 21 and secondary winding 24 in transmitter 15 still will present a relatively low impedance to 1000 c.p.s. energy so that suicient current from transmitter 10 will flow through rails 11a and 11b to maintain receiver 20 energized to keep its associated relay TR20 picked up. Filter 21 and secondary 24 in transmitter 10 will likewise effect a relatively low irnpedance electrical path for 1500 c.p.s. energy so that transmitter 15 can maintain receiver 25 energized. Thus, the circuit inherently provides a means of maintaining sufficient rail current flowing over any range of track ballast impedance that might be encountered. It will be understood that if the area to be controlled is a single direction track and only one transmitter and one receiver are used, a circuit tuned to the operating frequency may be coupled to the rails to provide a complete electrical circuit for the transmitter operating frequency.

The operating relation of the loops coupling signal energy from the rails to the receivers is similar, so that a description of receiver 20 and loop 16 is considered sufficient to the understanding of my invention. As noted above, loop 16 is located so that on an eastward move, toward the right as oriented in the drawings, the last axle of the train will have cleared the highway crossing before current flow ener-gizes receiver 20 to cause the associated relay TR20 to pick up to actuate the Crossing Gate Mechanism to its normal or raised position as shown in the drawings. The elements designated as G indicate the gates which are actuated by the Crossing Gate Mechanism. It has been found that under conditions of high battery voltage and infinite ballast impedance, suicient current flows in the rails when the last axle of the train causes a shunt at a point to the right of lead 16a a distance of about only 10% to 15% of the total distance between the leads 16a and 16b for energizing receiver 20 and picking up receiver relay TR20. Under conditions of low battery voltage and low ballast impedance, when the last axle of the train causes a shunt at a point to the right of lead 16a a distance of about 70% to 80% of the distance between leads 16a and 16b receiver 20 is energized sufhciently to pick up relay TR20. Therefore, the lead 16a that is the left hand end of loop 16 is positioned relative to the highway crossing so that under maximum voltage and infinite impedance ballast conditions, the train will always be off the highway before the loop couples suicient energy to the associated receiver to pick up its relay. However, the distance from the highway to receiver lead 16a is desirably as short as possible so that the period of time the Crossing Gate Mechanism is actuated down is held to a minimum after a train has passed the highway crossing 14.

The Crossing Gate Mechanism may be of any suitable known type and does not per se form a part of my invention.

FIGS. 2 and 3 show the details of the transmitter and receiver circuits which employ transistor devices. Transistors of the three-electrode PNP type are employed; however, transistors of the NPN type could likewise be employed by proper arrangement of the biasing potentials as is well-known to those skilled in the art.

The details of the transmitter circuit will now be described. Referring to FIG. 2, there is shown a transmitter oscillat-or or signal generator including transistor 31 which has a base 32, an emitter 33, and a collector 34. The co1- lector 34 is connected to a parallel resonant circuit 35 which determines the operating frequency of the oscillator and which consists of a capacitor 36 and the primary winding 37 of a transformer 38. As noted above, the oscillator of transmitter 10 is tuned to oscillate at 1000 c.p.s., while the oscillator of tr-ansmitter 15 is tuned to oscillate at 1500 c.p.s. The emitter 33 is connected through a feedback winding 41, resistance or resistor 42 and a signal current by-pass capacitor 43 to the base 32 to provide a feedback path to sustain oscillations. To provide proper biasing potentials for oscillator operation, the negative terminal of a suitable source of potential lindicated as a battery 44 is coupled through primary winding 37 to the collector 34z The positive terminal of source 44 is coupled through res1stors or resistances 47 and 42, and feedback winding 4I to the emitter 33. Base 32 is connected to the junction of resisto-rs 48 and 49, which resistors are connected across source 44.

One terminal of a secondary winding 39 of transformer 38 is connected to the emitter 52 of a transistor S3 which is a Class A amplifier. The other terminal of winding 39 is connected through resistors 60 and 77 to the positive terminal of source 44. A capacitor '79 connected in parallel to resistor 60 provides a signal current by-pass therefor. The junction of a pair of resistors 56 and 57 which are connected across source 44 is connected through lead 50a, secondary winding 58 of feedback transformer 28, lead 50h, and resistor 5l to the base 54 of transistor 53. The collector 55 of transistor 53 -is connected to one terminal of a tuned wave-shaping circuit consisting of a condenser 59 connected in parallel to the primary winding 6l of a transformer 62; the other terminal of the tuned circuit is connected to the negative terminal of source 44.

The secondary winding 63 of transformer 62 is connected to the base electrodes 65 and 72 of normally cutoff, push-pull connected transistors 64 and 71 which are amplifiers. One terminal of secondary winding 63 is connected to base 65 while its other terminal is connected to base 72. The emitters 66 and 73 of transistors 64 and 71 are directly connected to one another, and the collectors 67 and 74 are connected to opposite terminals of an output tuned circuit 76 consisting of capacitor 75 connected in parallel to the primary winding 26 of an -output transformer 27. A series circuit consisting of resistor 68, a thermistor 69 having a negative temperature coefiicient, and a resistor 70 is connected from a center tap on primary winding 26 of transformer 27 to the emitters 66 and 73 of transistors 64 and 71. To provide the proper operating or biasing potentials, the positive terminal of source 44 is connected through resistor 70 to emitters 66 and 73, and through thermistor 69 and a center tap on secondary winding 63 of transformer 62 to -bases 65 and 72. The negative terminal of source 44 is connected through the center tap on primary winding 26 of transformer 27 to collectors 67 and 74. As is known, the total impedance across the emitter to base of a transistor is inversely related to temperature. Thermistor 69 compensates for variations in temperature by controlling the bias potential applied to transistors 64 and 71 inversely with relation to temperature.

As noted above, secondary winding 24 and filter 21 couple the signal energy from the transmitter to the rails lla and 1lb. For simplicity, the total ballast or load impedance existing across the track rails llllzz and 11b is indicated as a single variable impedance ZL. It will be appreciated that the load impedance existing across the rails and presented to said transmitter consists essentially of an assumed plurality of rail impedances and parallel resistances as indicated in FIG. la.

The standards set forth by the various railroads for equipment to be used at highway crossings in which the rails have no insulated rail joints stipulate that the track relay operated from a receiver must in all cases pick up if the last vehicle of the train is some distance past the crossing. However, various state commerce commissions require that the said track relay must not pick up, or fail to release, if a train is standing in such a position that it is blocking the yhighway `and one pair of wheels is shunting the track rails with zero (0) ohms impedance at a point 100 feet distant on the opposite side of the crossing from the transmitter and another pair of wheels is shunting the track rails with 0.06 ohm impedance in the crossing itself, see FIG. 4c. It has been found that if a track circuit is in the order of 4000 feet long and a transmitter following a conventional or normal output regulation curve as shown in FIG. 4a is employed in the track circuit, the shunt combination mentioned in the previous sentence will not in all cases cause the track Irelay to release. This is due to the fact that in a track circuit in the order of 4000 feet operating at a frcqUSIlCY 0f 1500 c.p.s. and in the case of infinite ballast impedance conditions wherein each 1000 feet of rail provides 3 ohms irnpedance, a shunt 4000 feet from the transmitter will cause a 12-ohm load impedance to appear across the transmitter. Thus with a transmitter having conventional regulation as shown in FIG. 4a, the output voltage at the track will be relatively high and a shunt combination as discussed above and shown in FIG. 4c will not release the track relay.

In order to provide a means for assuring that the track relay will positively release under thev conditions mentioned above I have found it necessary to provide a novel transmitter following an output regulation curve as shown in FIG. 4b. To obtain a regulation as indicated in FIG. 4b, I provide a feedback voltage from the output circuit of the transmitter through feedback transformer 28 to the first or intermediate stage of amplification comprising transistor 53. The turns of secondary Winding 5S of transformer 28 are wound such `as to couple a voltage to the base 54 of transistor 53 which is of such a polarity and phase to aid or add to the signal voltage applied to emitter 52 of transistor 53 from the oscillator. This provides a regenerative, or positive feedback to transistor 53 which tends to increase the signal voltage. If the track load or Iballas-t indicated as impedance ZL is low, the current IL will be high, and the feedback voltage Vf developed across secondary winding 58 of transformer 28 will be relatively large. If the current IL is small due to a high ZL, the feedback voltage V, developed will be low. Thus the total input voltage to transistor 53 will be large when ZL is low and small when ZL is high. The voltage VL delivered to the track rails will follow the total input voltage to transistor 53, as shown in FIG. 4b.

As can be noted from FIG. 4b, the feedback resistor 51 connected in the base 54 of transistor 53 is selected and the circuit is adjusted to give maximum voltage output at about 2.25 ohms. This allows the output voltage at 1.5 ohms to equal the output at 3 ohms. This is desirable for it allows the transmitter to deliver the same output voltage to a track circuit having a 3.0 ohms per thousand feet impedance either by connecting the transmitter at the end of the track circuit or at the center of the track circuit. As is known, at finite ballast impedance conditions and for track circuits of 1000 feet and longer the circuits may be considered as infinitely long lines and the transmitter sees the characteristic impedance of the circuit which is determined largely by the ballast impedance.

Resistor 51 in conjunction with secondary winding 58 shifts the phase and magnitude of feedback voltage Vf so that it will be in proper phase relation to input signal voltage developed across secondary 39 of transformer 38. The loading resistor 78 is connected across primary winding 23 of transformer 28 to minimize the change in phase of the feedback voltage Vf so that one value of resistor 51 will suffice for diHerent load conditions presented by the track circuit. For example, a track load ZL will change phase depending on whether the track circuit is a center-fed, low ballast circuit, in which case the impedance might be 1.5 ohms at nearly unity power factor, or whether innite ballast conditions are present and a train shunt exists 500 feet from the transmitter, in which case the impedance might look like 1.5 ohms at 80 degrees. Loading resistor 78 tends to minimize any adverse effects due to this change in phase between the two aforementioned cases.

Feedback voltage Vf is large enough in proportion to the input signal voltage developed across secondary winding 39 of transformer 38 so that the transmitter output will fall to zero when the windings 23 or 58 of feedback transformer 28 are short circuited. Likewise the base 54 of transistor 53 will be open and the output voltage will be zero if the feedback windings 23 or 58 become open circuited. Thus, in either case transformer 28 is fail-safe. As is well known in the railway art, in this type of circuit the term fail-safe implies that a circuit or component is self checking and upon the failure of said circuit or component the track relay releases t give a danger indication.

The details of the receiver circuit will now be described. Referring to FIG. 3, each receiver includes a sharply tuned input circuit consisting of an inductance 82 and capacitor 81 connected in series to the primary winding 83 of transformer 84. The input circuit of receiver 20 and each amplifying stage therein is tuned to a frequency of 1000 cps while the input circuit of receiver 25 and each amplifying stage therein is tuned to a frequency of 1500 cps. Receivers 20 and 25 are sufiiciently sensitive to be energized as long as some finite value of current, in the order of 3 or 4 milliamperes at their respective tuned frequencies, is fiowing in that portion of rail to which loops 16 and 18 are inductively coupled.

Secondary winding 85 of transformer 84 is coupled to transistor 92 which operates as a Class A amplifier, and includes base 91, collector 93 and emitter 94 electrodes. As is known transformer 84 matches the input impedance of the transistor 92 to the impedance of the track rails. A parallel tuned circuit 87 consisting of a tertiary winding 88 on transformer 84 and capacitor 89 provides a sensitive means for selectively tuning the receiver input to the associated transmitter frequency while using a moderate size capacitor. One terminal of secondary winding 85 of transformer 84 connects to base 91, and its other or second terminal connects to the junction of two resistors 99 and 100 connected across a source of potential indicated as a battery 103. The collector 93 of transistor 92 couples through a parallel tuned circuit consisting of a capacitor 95 and the primary winding 96 of transformer 97 to the negative terminal of a source 103. The emitter 94 of transistor 92 couples through resistor 104 to the positive terminal of source 103.

A capacitor 101 is connected in series with resistors 86 and 102 from emitter 94 to the second terminal of winding 85. Resistors 86 and 102 provide a sensitivity control to adjust the signal input level to transistor 92. Resistor 102 is variable to permit fine adjustment control. By inserting the sensitivity control in the emitter 94 circuit, a safety feature is added to the circuit since if resistors 86 or 102 would now open the sensitivity of the receiver will be decreased to cause the associated track relay to be released to indicate a false condition and thus be fail-safe. The over-al1 sensitivity of the unit would be decreased because the capacitor 101 would no longer provide a by-pass for alternating current. The other possibility, that is, that resistors 86 and 102 will short circuit to zero impedance is believed to be remote. Since the resistances 86 and 102 are in the emitter circuit instead of, as is conventional, in the base circuit they have little effect on the direct current characteristics of the transistor. Removing the sensitivity control from the base 91 circuit tends to increase the ternperature stability of transistor 92 since resistors in said base circuit may cause thermal runaway at high operating temperatures. Further, had the input sensitivity control been connected in a conventional manner as a potentiometer across Winding 85 with the tap on the potentiometer being connected to the base circuit of transistor 92, and if a lead at one end of the potentiometer opened the receiver would become more sensitive. This would not be acceptable for railroad fail-safe type circuits.

The secondary winding 98 of transformer 97 connects in parallel to the bases 107 and 109 of pushpull connected Class B amplifiers 105 and 110, respectfully, which are biased to be slightly conducting during the period when no signal is being received in the receiver. The emitters 108 and 111 of amplifiers 105 and 110 are directly coupled to one another, and the collectors 106 and 112 of amplifiers 105 and 110 are o u coupled to opposite terminals of an output parallel tuned circiut consisting of capacitor 113 and the primary winding 114 of transformer 115.

The biasing or operating potential for amplifiers and is obtained from a source 103. The bases 107 and 109 are connected through a center tap on secondary 98 to the junction of a temperature compensating thermistor 117 having a negative temperature coefficient, and resistor 118. Emitters 108 and 111 are connected through resistor 119 to the positive terminal of source 103. Capacitor 121 provides a signal current by-pass for resistor 119. Collectors 106 and 112 are connected through a center tap of primary winding 114 of transformer 115 to the negative terminal of source 103. Thermistor 117 is connected in series with resistor 118, the two components then being connected across the terminals of source 103. The function of the thermistor 117 is to maintain a constant receiver sensitivity over a wide range of temperature, in one embodiment from 30 F. to +150 F. A constant receiver sensitivity is required for once the sensitivity is set it must remain at a fixed point for failsafe operation. Further, the circuit used to obtain this fixed sensitivity with respect to temperature must be self-checking so that if the thermistor 117 opens or shorts, the output will be reduced and the sensitivity decreased. The foregoing is accomplished as follows. As noted above, one terminal of thermistor 117 is connected in series to a resistor 118 which has a relatively low resistance, across source 103. Emitters 108 and 111 are connected through a resistor 119, which has a low power dissipation rating in one embodiment 0.5 watt, to the positive terminal of source 103, and to the other terminal of thermistor 117. If thermistor 117 becomes short circuited the forward bias in the pushpull stage will become zero, that is, the same relative potential will be applied to bases 107 and 109 as is applied to emitters 108 and 111 and the output signal Will be reduced markedly. 1f on the other hand, thermistor 117 opens and since the value of resistor 118 is selected to be relatively low, the bases 107 and 109 respectively will be approximately at the negative potential of source 103. Hence, the base current will become excessive, and as is known, the emitter to collector currents in amplifiers 105 and 110 will increase toward their maximum rated value. This causes resistor 119 to be destroyed or burnedup since it is not capable of handling the current now flowing in the emitter circuits. As soon as resistor 119 is destroyed current ow through amplifiers 105 and 110 will cease, causing the track relay to release.

The output of the receiver is taken from the secondary 116 of transformer 115 and coupled through a balanced rectifying device 122 to the associated track relay.

The receiver is rendered insensitive to 120 cycle alternating current ripple that may appear across the alternating current floating storage battery system as normally used in railway circuits. This is accomplished by tuning both transformers 97 and 115 to have a high vimpedance only to the frequency being passed. The primary and secondary inductances of transformers 97 and are chosen to give optimum selectivity in these circuits and toroidal coils are used throughout because of their inherently higher Q compared to conventional transformers. The presence of several tuning stages in the receiver circuit also precludes the possibility that the receiver may be tuned to the adjacent transmitter frequency through possible changes in tuning capacitor values.

The operation of the over-all svstem as shown in FIG. la will now be described. The operation of the circuits is bi-directional, that is, operation is similar for either a west to east movement, left to right as oriented on the drawings, or an east to west movement. Only a west to east movement will be described since this is deemed sufficient for an understanding of my track circuit.

In the track unoccupied condition, transmitter effects an alternating current flow in rails 11a and 11b at a frequency of 1000 c.p.s. and induces a voltage in loop 16 suiiicient to energize receiver 20 to keep its associated relay TR20 energized. As noted hereinbefore the output filter 21 and secondary Winding 24 of transformer 27 in transmitter 15 provide an electrical path across the rails 11a and 11b for the 1000 c.p.s. energy. Transmitter 15 oscillating at 1500 c.p.s. likewise energizes receiver 25 to keep relay TR25 energized since output filter and secondary winding of the output transformer in transmitter 10 provide an electrical path for 1500 c.p.s. energy.

Assuming now a train movement from west to east. A train axle, occupying an intermediate position between the point where leads 13a and 13b from transmitter 10 connect to rails 11a and 11b, and lead 16a, that is, the left hand end of loop 16, will effect a shunt across the rails to stop current ow in that portion of rails 11a and 11b adjacent to which loop 16 is positioned; thus, receiver will be deenergized. Accordingly, relay TR2() will release and interrupt the circuit extending from terminal B of a source of potential indicated as a battery 9 through front contact c of relay TR25, and front contact c of relay rTRZ through the energizing coil of crossing relay XR to terminal N. Interruption of the foregoing circuit will cause relay XR to release. Release of relay XR completes a circuit from terminal B through the Crossing Gate Mechanism and back contact a of relay XR to terminal N to actuate said mechanism, that is, to cause said mechanism to block the highway crossing. Release of relay TR20 also causes a circuit to be completed from terminal B, through back contact b of relay TRZO, back contact a of a slow release West Stick Relay WSR, and front contact a of relay TR through the energizing coil of slow release East Stick Relay ESR to terminal N to pick up the contacts of relay ESR. As the train axle moves to a position adjacent the point where loop 18 couples to rails 11a and 11b, the current normally flowing in that portion of the rails will be shunted by the train axle. Accordingly, receiver 25 will be deenergized and relay TR25 will release.

When the last train axle moves to the right of the highway to a predetermined position between the position of leads 16a and 161; as discussed above in relation to the fiexible placement of the receiver leads, receiver 20 will cause relay TR20 to be energized. The crossing relay XR will now be energized over the circuit extending from terminal B, through the front contact b of relay ESR, front Contact c of relay TR20, and the operating coil of relay XR to terminal N. When relay XR is energized, the circuit from terminal B through the Crossing Gate Mechanism and back contact a of relay XR to terminal N is interrupted, thus deactuating said Mechanism, that is, causing it to clear the highway at a relatively predetermined short distance after the train passes the highway crossing. Although the train axle Will still occupy a position between transmitter 15 and its associated receiver 25, relay ESR being initially energized over the circuit previously traced, remains energized over a stick circuit extending from terminal B, through back contact b of relay TR25, front contact a of relay ESR and the operating winding of relay ESR to terminal N. With relay ESR energized, the circuit extending from terminal B, through front contact b of relay ESR, and front contact c of relay TR2() and the operating winding of relay XR to terminal N keeps relay XR energized. When the train axle passes the transmitter 15 connections 17a and 17b, relay TR25 will be energized interrupting the stick circuit to relay ESR, as traced above, and relay ESR will release slowly. The crossing relay XR will, however, remain energized over a circuit extending from terminal B, front contact c of relay TR25, and front contact c of relay TR2@ through the operating coil of relay XR to terminal N.

While my invention has been described with reference to particular embodiments thereof, it will be understood that various modifications may be made by those skilled in the art Without departing from the invention. The appended claims are therefore intended to cover all such modifications within the true spirit and scope of the invention.

Having thus described my invention, what I claim is:

1. In a track circuit for a section of railway track a receiver of signal energy including a transistor input amplifier and a pair of transistor output amplifiers connected in push-pull, each of said transistor amplifiers including an emitter, a base, and a collector, said transistor amplifiers being susceptible to variations in operating characteristics due to temperature changes, means connecting said amplifiers to a direct current source of biasing potential, means coupling an input signal across the base to emitter of said input amplifier, means coupling the output signal from the collector of said input amplifier as an input signal to the bases of said output amplifiers, a sensitivity control circuit including at least one resistance and a capacitor serially connecting the emitter of said input amplifier to said input Signal coupling means, a fail-safe temperature compensating circuit for said output amplifiers including a thermistor having a negative temperature coefficient for compensating for variations due to changing operating characteristics of said output amplifiers, means connecting said thermistor and a second resistance across said direct current source for providing base-emitter biasing potential to said output amplifiers, a third resistance having a sufliciently low power dissipation rating to be burned-up by maximum current fioW through said output amplifiers, said emitters being connected to the positive terminal of said source through said third resistance, means connecting said collectors to the negative terminal of said source, means connecting the junction of said thermistor and said second resistance to the bases of said output amplifiers such that short circuiting of said thermistor decreases the base-emitter bias on said output amplifiers to decrease conduction therethrough and such that circuiting of said thermistor causes an increase in the base-emitter bias impressed on said output amplifiers to cause a momentarily increased conduction in said output amplifiers to burn-up said third resistance and thus reduce conduction in said output amplifiers.

2. A pair of push-pull connected transistor amplifiers which amplifiers are susceptible to variations in operating characteristics due to temperature changes, each of said amplifiers having emitter, base and collector electrodes, said emitters being connected directly to one another, means connecting said bases to a source of signal, means connecting a signal output from said collectors of said amplifiers, means connecting said amplifiers to a direct current source biasing potential, a fail-safe temperature compensating circuit including a thermistor having a negative temperature coefiicient for compensating for variations due to changing operating characteristics of said amplifiers, means connecting said thermistor and a first resistance across said direct current source for providing base-emitter biasing potential for said amplifiers, a second resistance having a sufficiently low power dissipation rating to be burned-up by maximum current fioW through said pair of amplifiers, said emitters being connected to the positive terminal of said source through said second resistance, means connecting said collectors to the negative terminal of said source, means connecting the junction of said thermistor and said first resistance to the bases of said amplifiers such that short circuiting of said thermistor decreases the base-emitter bias on said amplifiers to decrease conduction therethrough, and such that open circuiting of said thermistor causes an increase in the base-emitter bias impressed on said amplifiers to cause a momentarily increased condition in said amplifiers to burn-up said second resistance and thus reduce conduction in said output amplifiers.

3. An amplifier circuit arrangement comprising, a pair of transistors connected in push-pull, each of said transistors having an emitter, collector and base electrodes, means connecting a biasing potential to each of said transistors, means coupling an input signal to the base-emitter electrodes of each of said transistors, means for deriving an output signal from each of said transistors, a fail-safe temperature compensating circuit comprising a temperaturesensitive element having a negative temperature coeicient for minimizing variations in the output signal due to varying ambient temperature, means serially connecting said temperaturesensitive element and a first resistance across said biasing potential for providing base-emitter biasing potential for said transistors, a second resistance having a low power dissipation rating connecting the emitter of each transistor to the positive terminal of said biasing potential, means connecting each of said collectors to the negative terminal of said biasing potential, and means connecting the junction of said temperature-sensitive element in said first resistance to said base of each transistor in a manner such that during a short-circuited condition of said temperature-sensitive element the output signal is substantially reduced due to the decrease of the baseemitter biasing potential and such that during an opencircuited condition of said temperature-sensitive element the output signal momentarily increases due to the increase in the base-emitter potential and then ceases to ow due to the inherent self-destroying characteristics of said second resistance.

4. The arrangement as defined in claim 3, wherein said temperature-sensitive element comprises a thermistor.

5. A transistor amplifier including in combination, an input and an output amplifying unit, said input amplifying unit including a first transistor, said output amplifying unit including a second and third transistor connected in push-pull, each of said transistors having an emitter, co1- lector and base electrodes, means connecting a biasing potential to each of said transistors, input means connected to the emitter electrode of said first transistor such that the level of an input signal applied to the base and emitter electrodes of said first transistor is decreased during an open-circuited condition of said circuit, means for deriving an output signal from said emitter and collector electrodes of said first transistor, means coupling the output signal of said first transistor to the base and emitter electrodes of said second and third transistors, means for deriving an output signal from the collector electrodes of said second and third transistors, a fail-safe temperature compensating circuit comprising a temperature-sensitive element having a negative temperature coefficient for minimizing variations in said output signal due to varying ambient temperatures, means for serially connecting said temperature-sensitive element and a first resistance across said biasing potential, a second resistance having a low power dissipating rating connecting the emitter of said second and third transistors to the positive terminal of 15 said biasing potential, means connecting the collector of said second and third transistors to the negative terminal of said biasing potential, and means connecting the junction of said temperature sensitive element and said first resistance to the bases of said second and third transistors in a manner such that during a short-circuited condition of said temperature-sensitive element the output signal is substantially reduced due to the decrease of biasing potential and such that during an open-circuited condition of said temperature-sensitive element the output signal ceases to ow due to the inherent self-destroying characteristic of said second resistance.

6. The combination as defined in claim 5, wherein said sensitivity control circuit comprises a capacitor and at least one resistor.

7. The combination as defined in claim 5, wherein said temperature-sensitive element comprises a thermistor.

References Cited by the Examiner ROY LAKE, Primary Examiner.

NATHAN KAUFMAN, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3 223 ,Q34 December 14 1965 Philip H. Luft at error appears in the above numbered pat- It is hereby certified th Letters Patent should read as ent requiring correction and that the said corrected below.

Column IO, line 40, after "that" insert open Column ll line 5 8, aftelmmeans" insert including a Sensitivity control Circuit Signed and sealed this 10th day of May 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER 

3. AN AMPLIFIER CIRCUIT ARRANGEMETN COMPRISING, A PAIR OF TRANSISTORS CONNECTED IN PUSH-PULL, EACH OF SAID TRANSISTORS HAVING AN EMITTER, COLLECTOR AND BASE ELECTRODES, MEANS CONNECTING A BIASING POTENTIAL TO EACH OF SAID TRANSISTORS, MEANS COUPLING AN INPUT SIGNAL TO THE BASE-EMITTER ELECTRODES OF EACH OF SAID TRANSISTORS, MEANS FOR DERIVING AN OUTPUT SIGNAL FROM EACH OF SAID TRANSISTORS, A FAIL-SAFE TEMPERATURE COMPENSATING CIRCUIT COMPRISING A TEMPERATURSENSITIVE ELEMENT HAVING A NEGATIVE TEMPERATURE COEFFICIENT FOIR MINIMIZING VARIATIONS IN THE OUTPUT SIGNAL DUE TO VARYING AMBIENT TEMPERATURE, MEANS SERIALLY CONNECTING SAID TEMPERATURE-SENSITIVE ELEMENT AND A FIRST RESISTANCE ACROSS SAID BIASING POTENTIAL FOR PROVIDING ABASE-EMITTER BAISING POTENTIAL FOR SAID TRANSISTORS, A SECOND RESISTANCE HAVING A LOW POWER DISSIPATION RATING CONNECTING THE EMITTER OF EACH TRANSISTOR TO THE POSITIVE TERMINAL OF SAID BIASING POTENTIAL, MEANS CONNECTING EACH OF SAID COLLECTORS TO THE NEGATIVE TERMINAL OF SAID BIASING POTENTIAL, AND MEANS CONNECTING THE JUNCTION OF SAID TEMPERATURE-SENSITIVE ELEMENT IN SAID FIRST RESISTANCE TO SAID BASE OF EACH TRANSISTOR IN A AMNNER SUCH THAT DURING A SHORT-CIRCUITED CONDITION OF SAID TEMPERATURE-SENSITIVE ELEMENT THE OUTPUT SIGNAL IS SUBSTANTIJALLY REDUCED DUE TO THE DECREASE OF THE BASEEMITTER BIASING POTENTIAL AND SUCH THAT DURING AN OPENCIRCUITED CONDITION OF SAID TEMPERATURE-SENSITIVE ELEMENT THE OUTPUT SIGNAL MOMENTARILY INCREASES DUE TO THE INCREASE IN THE BASE-EMITTER POTENTIAL AND THEN CEASES TO FLOW DUE TO THE INHERENT SELF-DESTROYING CHARACTERISTICS OF SAID SECOND RESISTANCE. 